Small Wind Turbine Power Output Calculator: Rotor Diameter & Betz Law
Small wind turbine marketing wattage ratings are usually measured at high wind speeds rarely seen at most sites, so real-world output at your actual average wind speed can look very different. This calculator applies the Betz Limit physics formula to your rotor diameter, local wind speed and generator efficiency to estimate realistic watts and monthly kWh.
At 10.0 mph average wind, this rotor produces roughly 30W continuous, or about 21.5 kWh/month.
| Measure | Value |
|---|---|
| Rotor diameter | 6.0 ft (1.83 m) |
| Wind speed | 10.0 mph (4.47 m/s) |
| Swept area | 2.63 m² |
| Air density | 1.225 kg/m³ |
| Theoretical power (Betz) | 85 W |
| Generator efficiency | 35% |
| Actual output | 30 W |
| Monthly energy | 21.5 kWh |
- 1.
Swept_Area = π × (Diameter_m / 2)² - 2.
Theoretical_Power_W = 0.5 × Air_Density × Swept_Area × WindSpeed_mps³ × 0.593 - 3.
Actual_Output_W = Theoretical_Power_W × Efficiency%
How to Use This Calculator
1. Enter your details
Use the labelled fields and units that match your situation.
2. Calculate instantly
The result, visual comparison, and working update as values change.
3. Review your result
Check assumptions, then copy, print, or share the calculation.
Formula & step-by-step maths
- 0.593
- Betz Limit maximum extractable fraction
- WindSpeed_mps
- Wind speed converted to meters/second
The Betz Limit and why it caps turbine efficiency
German physicist Albert Betz proved in 1919 that no wind turbine can capture more than 59.3% of the kinetic energy in moving air, because some airflow must continue past the rotor to avoid a complete stall. This calculator applies that 0.593 constant to theoretical power.
Why wind speed matters cubically
Power scales with the cube of wind speed, meaning doubling wind speed from 5 to 10 mph increases available power eight-fold, not just double. This is why turbine siting on the windiest available spot matters far more than most other design choices.
Real generator efficiency losses
Beyond the Betz Limit, real turbines lose additional energy to generator electrical resistance, gearbox friction (if geared) and inverter conversion — small residential turbines typically achieve only 25-40% overall efficiency versus the theoretical Betz maximum.
Realistic expectations for small wind at home
Small rooftop or pole-mounted turbines rarely produce meaningful power below about 8-10 mph average wind speed; sites without consistent wind above this threshold generally see better returns from solar panels instead.
Estimated output by rotor diameter at 10mph wind
| Rotor diameter | Swept area | Approx. output (35% eff.) |
|---|---|---|
| 3 ft | 0.66 m² | ~14 W |
| 6 ft | 2.64 m² | ~57 W |
| 8 ft | 4.68 m² | ~101 W |
| 10 ft | 7.31 m² | ~158 W |
| 12 ft | 10.5 m² | ~228 W |
Frequently Asked Questions
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: rotor blade diameter 4.68 ft
On the lower / more conservative end. At 10.0 mph average wind, this rotor produces roughly 18W continuous, or about 13.1 kWh/month.
Example 2: rotor blade diameter 6 ft
A typical middle-of-the-road setup. At 10.0 mph average wind, this rotor produces roughly 30W continuous, or about 21.5 kWh/month.
Example 3: rotor blade diameter 7.8 ft
On the higher / more demanding end. At 10.0 mph average wind, this rotor produces roughly 50W continuous, or about 36.3 kWh/month.
Quick answers about the Wind Turbine Power Output
What exactly does the Wind Turbine Power Output work out?
Small wind turbine marketing wattage ratings are usually measured at high wind speeds rarely seen at most sites, so real-world output at your actual average wind speed can look very different. You enter rotor blade diameter, average local wind speed, air density and generator electrical efficiency and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 4 fields: rotor blade diameter, average local wind speed, air density and generator electrical efficiency. Nothing else is needed and nothing is stored.
How is it calculated — the Betz Limit and why it caps turbine efficiency?
German physicist Albert Betz proved in 1919 that no wind turbine can capture more than 59. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for wind Turbine Power Output?
Power scales with the cube of wind speed, meaning doubling wind speed from 5 to 10 mph increases available power eight-fold, not just double. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Estimated output by rotor diameter at 10mph wind" table on this page tell me?
It is the reference range this tool works against — 5 rows from "3 ft" (0.66 m²) up to "12 ft" (10.5 m²). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Do I have to press a button or reload the page to see the result?
No. Wind Turbine Power Output runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Wind Turbine Power Output leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
Most 400W-rated small turbines are rated at much higher wind speeds (often 25-28 mph); at 10 mph, real output is typically only 40-80W depending on rotor size and efficiency. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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